Corrosion-resistant trivalent-chromium chemical conversion coating and solution for trivalent-chromium chemical treatment

ABSTRACT

A trivalent-chromium chemical conversion coating from which substantially no hexavalent chromium is released. The trivalent-chromium chemical conversion coating is one formed on the surface of a zinc or zinc-alloy deposit. In a brine spray test, the chemical conversion coating has unsusceptibility to corrosion (time required for white-rust formation) of 96 hours or longer. The chemical conversion coating has a hexavalent-chromium concentration less than 0.01 μg/cm in terms of metal atom amount. The amount of hexavalent chromium released after 30-day standing in a thermo-hygrostatic chamber at a temperature of 80° C. and a humidity of 95% (amount of hexavalent chromium released when the coating is immersed in 100° C. water for 10 minutes) is smaller than 0.05 μg/cm 2 .

TECHNICAL FIELD

The present invention relates to a trivalent chromium corrosionresistant chemical conversion treatment coating film from whichhexavalent chromium is not substantially eluted, the coating film beingformed on zinc plating or zinc alloy plating, and a trivalent chromiumchemical conversion treatment solution and a post-treatment solutionafter chemical conversion treatment used to form such a chemicalconversion treatment coating film.

BACKGROUND ART

A method using zinc plating has been relatively widely employed as amethod for inhibiting corrosion of the surface of a metal. However, theplating by itself does not provide a sufficient corrosion resistance,and thus a chromate treatment using hexavalent chromium after platinghas been widely employed in industry. However, it has been pointed outin recent years that hexavalent chromium harms human bodies and theenvironment, and, as a result, the use of hexavalent chromium has beenregulated.

In this connection, a trivalent chromium chemical conversion coatingfilm using trivalent chromium has been developed as an alternativetechnology to a coating film formed with hexavalent chromium, and hasstarted to be used. For example, Japanese Patent Application PublicationNo. 2000-509434 discloses a treatment method in which 5 to 100 g/L oftrivalent chromium, nitrate, an organic acid and a salt of a metal suchas cobalt are used. This method is to obtain a good corrosion resistanceby carrying out the treatment at an elevated temperature in highconcentration of chromium to form a thick trivalent chromium chemicalconversion coating film. However, the method has disadvantage inwastewater treatment, because the concentration of chromium and theconcentration of the organic acid in the treatment bath are high.

-   Patent Article 1: Japanese Patent Application Publication No.    2000-509434

DISCLOSURE OF THE INVENTION

In addition, it has been found out that there is a problem that, whensuch a conventional hexavalent chromium-free trivalent chromium chemicalconversion treatment coating film is left for a long period in a naturalenvironment, trivalent chromium in the coating film is oxidized, andharmful hexavalent chromium is detected in the coating film.

An object of the present invention is to provide a trivalent chromiumcorrosion resistant chemical conversion treatment coating film beingformed on zinc or zinc-based alloy plating, and having a corrosionresistance equal to or more than a conventional coating film with a lowchromium content. Moreover, in consideration of effect on human bodiesand the environment, hexavalent chromium is not substantially detectedin the coating film even after the coating film is left. In addition,another object of the present invention is to provide a trivalentchromium chemical conversion treatment solution and a post-treatmentsolution used after chemical conversion treatment, the treatmentsolution and the post-treatment solution being capable of forming such achemical conversion treatment coating film.

The present inventors have made a thorough examination and found that achemical conversion coating film that has a high corrosion resistanceand from which hexavalent chromium is substantially not eluted evenafter the coating film is left can be obtained from a trivalent chromiumchemical conversion coating film having low trivalent chromiumconcentration if the coating film itself is provided with a function ofsuppressing oxidation from trivalent chromium in the coating film tohexavalent chromium. In addition, the present inventors also found thatsuch a chemical conversion coating film can be obtained by using achemical conversion treatment liquid having a specific composition.

Accordingly, the present invention is a trivalent chromium chemicalconversion coating film formed on a surface of zinc or zinc alloyplating characterized in that a corrosion resistance (a time requiredfor the formation of white rust) in a salt spray test is 96 hours ormore, a concentration of hexavalent chromium in terms of metal atoms inthe chemical conversion coating film is less than 0.01 μg/cm², and anamount of hexavalent chromium eluted from the coating film left for 30days in a constant temperature and humidity chamber at a temperature of80° C. and at a humidity of 95% (an amount eluted by immersion of thecoating film into hot water at a temperature of 100° C. for 10 minutes)is less than 0.05 μg/cm².

A preferred aspect of the present invention is the above-descriedtrivalent chromium chemical conversion coating film, in which aconcentration of trivalent chromium in terms of metal atoms in thechemical conversion coating film is 2 to 20 μg/cm².

In addition, the trivalent chromium chemical conversion coating film, inwhich a cobalt concentration in the chemical conversion coating film is0.2 to 3.5 μg/cm², is an aspect of the present invention. In thisaspect, a preferable cobalt concentration is 0.3 to 3 μg/cm².

Furthermore, the trivalent chromium chemical conversion coating film, inwhich a cobalt concentration in the chemical conversion coating film isless than 0.2 μg/cm², is an aspect of the present invention. In thisaspect, a preferable cobalt concentration is 0 to 0.17 μg/cm².

The reason why the chemical conversion coating film that have a highcorrosion resistance and from which hexavalent chromium is substantiallynot eluted even after the coating film is left can be obtained byproviding the coating film itself with a function of suppressingoxidation from trivalent chromium in the coating film to hexavalentchromium is not clear. However, through the investigation in the presentinvention, the present inventors assume that the reason is as follows.Specifically, it is assumed that detection of hexavalent chromium from agenerally used trivalent chromium chemical conversion coating film whichis being left is caused because Co³⁺ in the chemical conversion coatingfilm acts as an oxidizing agent to oxidize trivalent chromium.

Formation of a trivalent chromium chemical conversion coating film andan assumed generation mechanism of Cr⁶⁺ by action of Co³⁺ will bedescribed below.

(i) Zinc is dissolved in an acidic treatment liquid, and electrons arereleased.

Zn→Zn²⁺+2e ⁻

(ii) Hydrogen ions are consumed at the interface between the zinc andthe treatment liquid, and the pH of the treatment liquid rises.

2H⁺+2e ⁻→H_(2↑)

(iii) Chromium hydroxide is generated by the rise of pH of the treatmentliquid.

2Cr³⁺+6(OH⁻)→2Cr(OH)_(3↑)

(iv) Also from Co²⁺ that is used for improving the corrosion resistancein the treatment liquid, cobalt(II) hydroxide is generated in thevicinity of the interface with the zinc with the rise of pH. However,Co²⁺ is converted to stable Co³⁺ with time, because Co²⁺ is unstable onthe alkaline side.

Co(OH)₂→Co(OH)₃

(v) Other insoluble substances (SiO₂) and a small amount of thetreatment liquid are also adsorbed and impregnated to be taken into thecoating film, with gelation and deposition of chromium hydroxide andcobalt hydroxide (Co²⁺, Co³⁺).(vi) The coating film is hardened in drying processes by deposition,dewatering of adsorbed substances and solidification. However, thecoating film is not hardened when the drying is insufficient, and it isexpected that chemical reactions will proceed in the coating film.(vii) Since the chemical conversion coating film containing thetreatment liquid is in a slightly acidic atmosphere, cobalt (III)hydroxide in the coating film is gradually liberated and dissolved, andCo³⁺ is converted to Co²⁺ that is stable in an acidic condition. Inaddition, it is considered that chromium hydroxide is also liberated anddissolved to cause the following reactions.

3Co³⁺+3e ⁻→3Co²⁺

Cr³⁺→Cr⁶⁺+3e ⁻

Combination of these formulae gives the following.

Cr³⁺+3Co³⁺→Cr⁶⁺+3Co²⁺

In short, it is considered that trivalent cobalt in the coating filmoxidizes trivalent chromium to generate hexavalent chromium.

Meanwhile, it is considered that, even when Co³⁺ is not contained oreven when the coating film is formed from a chemical conversiontreatment liquid having a strong oxidizing effect caused by acombination of chloric acid-nitric acid, or the like, hexavalentchromium is generated. Therefore, it is assumed that reduction ofnitrate ion concentration in the treatment liquid is also helps tosuppress the generation of hexavalent chromium. It is considered that,in a chemical conversion coating film containing a manganese compoundsuch as manganese dioxide, and in a chemical conversion coating filmcontaining, in the coating film, a large amount of ions of an elementother than Co whose valence can vary, hexavalent chromium is generatedby oxidation of trivalent chromium in a similar manner.

In addition, the present invention is achieved on the basis of thefollowing discovery. Specifically, by adding a hexavalent chromiumgeneration suppressing agent having an effect of suppressing hexavalentchromium generation to any one of a trivalent chromium chemicalconversion treatment liquid, washing water for a trivalent chromiumchemical conversion coating film and a finishing liquid therefor,hexavalent chromium which is otherwise generated in the trivalentchromium chemical conversion coating film can be suppressed, and anamount of hexavalent chromium eluted from the trivalent chromiumchemical conversion coating film (an amount eluted when the coating filmis immersed in a hot water at a temperature of 100° C. for 10 minutes)can be less than 0.05 μg/cm² even after the coating film is left.

Therefore, the present invention provides a chemical conversiontreatment liquid for forming, on zinc or zinc alloy plating, a trivalentchromium chemical conversion coating film from which hexavalent chromiumis substantially not eluted even after the coating film is left. Thechemical conversion treatment liquid is characterized in that a contentof trivalent chromium ions in the treatment liquid is 0.002 to 0.5mol/l, a concentration of hexavalent chromium ions is 1 ppm or less, acontent of cobalt ions is 0.1 mol/l or less, a hexavalent chromiumgeneration suppressing agent that can suppress generation of hexavalentchromium which is generated in the trivalent chromium chemicalconversion coating film is contained in said liquid, and a pH of thechemical conversion treatment liquid is 0.5 to 5.

In addition, the present invention provides washing water or a finishingliquid for a trivalent chromium chemical conversion coating film formedon zinc or zinc alloy plating, for suppressing hexavalent chromium whichis otherwise generated in the trivalent chromium chemical conversioncoating film. The washing water or the finishing liquid is characterizedby containing a hexavalent chromium generation suppressing agent thatcan suppress generation of hexavalent chromium which is generated in thetrivalent chromium chemical conversion coating film at 0.1 to 10 g/l,and characterized in that a pH of the washing water or the finishingliquid is 2 to 10.

In addition, the present invention is achieved on the basis of thefollowing discovery. Specifically, a trivalent chromate coating film isformed by use of a trivalent chromium chemical conversion treatmentliquid in which a content of cobalt ions is 250 ppm or less and acontent of a sulfur compound is in the range of 100 to 1000 ppm in termsof sulfur atoms. As a result, hexavalent chromium which is otherwisegenerated in the trivalent chromium chemical conversion coating film canbe suppressed, and an amount of hexavalent chromium eluted from thetrivalent chromium chemical conversion coating film can be less than0.05 μg/cm² even after the coating film is left (an amount eluted whenthe coating film is immersed in hot water at a temperature of 100° C.for 10 minutes).

Therefore, the present invention provides a chemical conversiontreatment liquid for forming a trivalent chromate coating film fromwhich hexavalent chromium is not substantially eluted after the coatingfilm is left, the trivalent chromate coating film being formed on zincor zinc alloy plating. The chemical conversion treatment liquid ischaracterized in that a content of trivalent chromium ions in thetreatment liquid is 0.002 to 0.5 mol/l, a concentration of hexavalentchromium ions is 1 ppm or less, a content of cobalt ions is 250 ppm orless, and a sulfur compound is contained in the range of 100 to 1500 ppmin terms of sulfur atoms.

The trivalent chromium chemical conversion coating film according to thepresent invention further has an excellent corrosion resistance of thetrivalent chromium chemical conversion coating film, in addition to acorrosion resistance of zinc plating itself. In addition, the coatingfilm obtained by forming the trivalent chromium chemical conversioncoating film directly on zinc plating undergoes no substantial elutionof hexavalent chromium after the coating film is left, has a corrosionresistance and a salt water resistance equal to or higher than those ofconventional hexavalent chromate, and can be applied in various colors.In addition, in the chemical conversion treatment liquid according tothe present invention that can form such a chemical conversion coatingfilm, the trivalent chromium concentration in the treatment liquid islow, and an organic acid concentration or nitrogen concentration canalso further be reduced. Therefore, the treatment liquid is advantageousin wastewater treatment and thus has excellent cost performance.

BEST MODE FOR CARRYING OUT THE INVENTION

The substrate used in the present invention may be made of any of thefollowing materials: various metals such as iron, nickel and copper;alloys thereof; and metals and alloys such as aluminum, which have beensubjected to zincate conversion treatment, and may have any of variousshapes such as plate-like, rectangular, column-like, cylindrical andspherical shapes.

The above substrate is plated with zinc or a zinc alloy by the usualmethod. The zinc plating may be deposited on the substrate using eitherof the following baths: an acidic/neutral bath such as a sulfuric acidbath, a borofluoride bath, a potassium chloride bath, a sodium chloridebath or an ammonium chloride-potassium chloride bath; or an alkalinebath such as a cyanide bath, a zincate bath or a pyrophoric acid bath,but particularly, a cyanide bath is preferable. The zinc alloy platingmay be performed using either an ammonium chloride bath or an alkalinebath such as an organic chelate bath.

In addition, the zinc alloy plating may be zinc-iron alloy plating,zinc-nickel alloy plating, zinc-cobalt alloy plating or tin-zinc alloyplating. Zinc-iron alloy plating is preferable. The zinc or zinc alloyplating may be deposited on a substrate in any thickness, but preferablyin a thickness of 1 μm or more, and more preferably in a thickness of 5to 25 μm.

In the present invention, after the zinc or zinc alloy plating isdeposited on a substrate according to the above method, the platedsubstrate is appropriately pretreated by, for example, being washed withwater and optionally activated by a nitric acid, as needed. Thereafter,the zinc or zinc alloy plating is subjected to chemical conversiontreatment by a dipping treatment, or the like using a chemicalconversion treatment liquid for forming the trivalent chromium chemicalconversion coating film according to the present invention.

The chemical conversion treatment liquid of the first aspect of thepresent invention contains trivalent chromium ions, cobalt ions andhexavalent chromium generation suppressing agent that can suppressgeneration of hexavalent chromium which is otherwise generated in thetrivalent chromium chemical conversion coating film.

In the chemical conversion treatment liquid, any chromium compoundcontaining trivalent chromium ions may be used as a source of trivalentchromium ions. For example, the sources of trivalent chromium salts suchas chromium chloride, chromium sulfate, chromium nitrate, chromiumphosphate or chromium acetate can be used, or, alternatively, trivalentchromium ions can be obtained by the reduction of hexavalent chromiumions of chromic acid, a dichromate, or the like with a reducing agent.However, the sources are not limited to these examples. One of the abovesources of trivalent chromium ions or any combination of at least two ofthem may be used. A content of trivalent chromium ions in the chemicalconversion treatment liquid should preferably be 0.002 to 0.5 mol/l, andshould more preferably be 0.02 to 0.1 mol/l. Meanwhile, a concentrationof hexavalent chromium ions in the chemical conversion treatment liquidshould preferably be 1 ppm or less, and should more preferably be 0.5ppm or less.

In the chemical conversion treatment liquid, any metal compoundcontaining cobalt may be used as a source of cobalt ions. Examples ofsuch metal compounds include cobalt nitrate, cobalt sulfate, cobaltchloride, cobalt carbonate and cobalt hydroxide. However, the metalcompounds are not limited to these examples. One of the above metalcompounds or any combination of at least two of them may be used. Acontent of cobalt ions in the chemical conversion treatment liquidshould preferably be 0.1 mol/l or less, should more preferably be 0.001to 0.06 mol/l, and should still more preferably be 0.005 to 0.04 mol/l.

In the chemical conversion treatment liquid, any additives can be usedas the hexavalent chromium generation suppressing agent as long as theadditives can suppress generation of hexavalent chromium which isotherwise generated in the trivalent chromium chemical conversioncoating film. In order to find out additives that can suppressgeneration of hexavalent chromium, various additives are added tochemical conversion treatment liquids for forming the trivalent chromatecoating film and effect of the additives is examined by experiments. Asa result, organic reducing compounds such as ascorbate ions, citrateions, tannate ions, gallate ions, tartrate ions, hydroxy(iso)quinolines,phenols and thiourea, and inorganic or metal reducing compounds such asphosphate ions, chromium phosphate ions, vanadium ions and titanium ionsshow the effect. Therefore, preferable hexavalent chromium generationsuppressing agents include ascorbic acid, salts thereof, citric acid,salts thereof, tannic acid, salts thereof, gallic acid, salts thereof,tartaric acid, salts thereof, thiourea, phosphoric acid, salts thereof,vanadium compounds, titanium compounds, and the like. A content of thehexavalent chromium generation suppressing agent in the chemicalconversion treatment liquid should preferably be 0.1 to 5 g/l, shouldmore preferably be 0.2 to 3 g/l, and should still more preferably be 0.3to 2 g/l.

A pH of the chemical conversion treatment liquid should preferably be0.5 to 5, and should more preferably be 2 to 3. The pH can be adjustedto this range by using the inorganic acid ions as described below, andalso by using an alkaline agent such as an alkaline hydroxide, ammoniawater, or the like.

The chemical conversion treatment liquid may contain one or more kindsselected from inorganic acids, alkaline salts thereof, and the like.Examples of inorganic acids include sulfuric acid, nitric acid,hydrochloric acid, and the like. However, the inorganic acids are notlimited to these examples. When one or more kinds selected frominorganic acids, alkaline salts thereof, and the like are contained, aconcentration thereof in the chemical conversion treatment liquid shouldpreferably be 1 to 50 g/L, and should more preferably be 4 to 20 g/L.

In addition, the chemical conversion treatment liquid may contain one ormore kinds of hydroxycarboxylic acids, monocarboxylic acids, polyvalentcarboxylic acids, aminocarboxylic acids, alkaline salts thereof, and thelike as chelating agents for trivalent chromium ions. Examples ofhydroxycarboxylic acids include malic acid, and the like. However, thehydroxycarboxylic acids are not limited to these examples. Examples ofmonocarboxylic acids include formic acid, acetic acid, and the like.However, the monocarboxylic acids are not limited to these examples.Examples of polyvalent carboxylic acids include: dicarboxylic acids suchas oxalic acid, malonic acid, succinic acid, adipic acid and diglycolicacid; tricarboxylic acids such as propanetricarboxylic acid, and thelike. However, the polyvalent carboxylic acids are not limited to theseexamples. Examples of aminocarboxylic acids include glycine, asparticacid, and the like. However, the aminocarboxylic acids are not limitedto these examples. Among these, polyvalent carboxylic acids arepreferable, and oxalic acid, malonic acid and succinic acid are morepreferable. When the chelating agent for trivalent chromium ions iscontained, a concentration thereof in the chemical conversion treatmentliquid should preferably be in the range of 0.2 to 2 mole, should morepreferably be in the range of 0.3 to 2 mole, should still morepreferably be in the range of 0.5 to 2 mole, and should still furthermore preferably be in the range of 0.7 to 1.8 mole per mole of trivalentchromium ions.

In addition, the chemical conversion treatment liquid may contain one ormore kinds of silicon compounds. Examples of silicon compounds includecolloidal silica, sodium silicate, potassium silicate, lithium silicate,and the like. However, the silicon compounds are not limited to theseexamples. When the silicon compound is contained, a concentrationthereof in the chemical conversion treatment liquid should preferably be1 to 20 g/l, and should more preferably be 2 to 10 g/l in terms of Si.Colloidal silica is particularly preferable. A concentration thereofshould preferably be 1 to 100 ml/l as a 20% SiO2 aqueous solution. Byadding colloidal silica, a coating film with a bilayer structure of aSi—O layer and a Cr—O layer can be formed, whereby the corrosionresistance can be further improved.

In addition, the chemical conversion treatment liquid may contain one ormore kinds of agents for reducing a coating film overall frictioncoefficient. Examples of such agents for reducing a coating film overallfriction coefficient include quinoline-based compounds such as quinolinesulfonic acid, quinaldic acid, quinophthalone and derivatives thereofdescribed in Japanese Patent Application Publication No. 2005-248233.When the agent for reducing a coating film overall friction coefficientis contained, a concentration thereof in the chemical conversiontreatment liquid should preferably be 0.1 to 25 g/l, and should morepreferably be 0.2 to 15 g/l. The trivalent chromium chemical conversioncoating film according to the present invention formed by treatment withthe chemical conversion treatment liquid containing such a agent forreducing a coating film overall friction coefficient is a coating filmthat has a reduced coating film overall friction coefficient.

The rest of the chemical conversion treatment liquid other than theabove essential components is water.

Usually, a nitrogen-containing compound, mainly nitrate ions, is used inlarge amount in a trivalent chromium chemical conversion treatmentliquid, for improving the corrosion resistance of a trivalent chromiumchemical conversion coating film. Consequently, the nitrogen atomconcentration in the treatment liquid is high, for example 3 to 9 g/l,and there is a problem in terms of the wastewater treatment. In thechemical conversion treatment liquid according to the present invention,nitrate ions may be used in an amount similar to a conventionaltreatment liquid. However, even when nitrate ions are considerablydecreased, and a nitrogen atom concentration, in the chemical conversiontreatment liquid, mainly derived from nitrate ions is considerablyreduced to 500 ppm/1 or less, the trivalent chromium chemical conversioncoating film that has an excellent corrosion resistance and from whichelution of hexavalent chromium is suppressed when the coating film isleft can be obtained from the chemical conversion treatment liquid. Inthe above treatment liquid, a content is specifically 500 ppm or less interms of nitrogen atoms, should preferably be in the range of 30 to 400ppm and should more preferably be in the range of 50 to 300 ppm, forexample. A metal reducing compound is preferable as the hexavalentchromium generation suppressing agent. Particularly, vanadium compounds,titanium compounds, magnesium compounds and combination thereof arepreferable.

In addition, cobalt ions may be contained but are not necessarilycontained. However, cobalt ions should be contained preferably in therange of 0.001 to 0.06 mol/l, and more preferably in the range of 0.005to 0.04 mol/l, because corrosion resistance of the chemical conversioncoating film under heating is further improved.

A method for forming a trivalent chromium chemical conversion coatingfilm on zinc or zinc alloy plating by using the chemical conversiontreatment liquid is commonly to immerse a zinc or zinc alloy platedsubstrate into the chemical conversion treatment liquid. A temperatureof the chemical conversion treatment liquid at immersion is, forexample, 10 to 70° C. The temperature should preferably be 30 to 50° C.An immersion time should preferably be 5 to 600 seconds, and should morepreferably be 15 to 120 seconds. Meanwhile, immersion into a dilutednitric acid solution, a diluted sulfuric acid solution, a dilutedhydrochloric acid solution, a diluted hydrofluoric acid solution, or thelike may be performed before trivalent chromium chemical conversiontreatment, for activating the surface of the zinc or zinc alloy plating.The conditions and treatment operations other than those described abovemay follow the conventional hexavalent chromate treatment method.

Meanwhile, the second aspect of the present invention is washing wateror a finishing liquid for a trivalent chromium chemical conversioncoating film formed on zinc or zinc alloy plating. The washing water orthe finishing liquid is used for suppressing hexavalent chromium whichis otherwise generated in the trivalent chromium chemical conversioncoating film. The washing water or the finishing liquid contains ahexavalent chromium generation suppressing agent that can suppressgeneration of hexavalent chromium which is otherwise generated in thetrivalent chromium chemical conversion coating film. A method forforming a trivalent chromium chemical conversion coating film to whichthe washing water or the finishing liquid is applied is not particularlylimited, and may be any known method. The washing water or the finishingliquid is particularly effective in the following cases: the trivalentchromium chemical conversion coating film contains Co²⁺ and Co³⁺; thetrivalent chromium chemical conversion coating film is a coating filmformed from a chemical conversion treatment liquid having a strongeroxidizing effect caused by a combination of chloric acid-nitric acid, orthe like; the trivalent chromium chemical conversion coating filmcontains a manganese compound such as manganese dioxide; and thetrivalent chromium chemical conversion coating film contains, in thecoating film, a large amount of ions of an element, other than Co, whosevalence can vary. Meanwhile, the hexavalent chromium generationsuppressing agent is already described above. A content of thehexavalent chromium generation suppressing agent in the washing water orthe finishing liquid should preferably be 0.1 to 10 g/l, should morepreferably be 0.2 to 5 g/l, and should still more preferably be 0.3 to 3g/l. Examples of preferable hexavalent chromium generation suppressingagents include ascorbic acid, salts thereof, citric acid, salts thereof,tannic acid, salts thereof, gallic acid, salts thereof, tartaric acid,salts thereof, thiourea, phosphoric acid, salts thereof, and the like.

A pH of the washing water or the finishing liquid should preferably be 2to 10, and should more preferably be 3 to 6. The pH may be adjusted tothis range by using the inorganic acid ions as described below, or byusing an alkaline agent such as an alkaline hydroxide, ammonia water, orthe like.

A method for treating a trivalent chromium chemical conversion coatingfilm using the washing water or the finishing liquid is not particularlylimited, and conventional and known methods such as immersing, applying,spraying, and the like may be used. However, to immerse a trivalentchromium chemical conversion coating film into the washing water or thefinishing liquid is commonly employed. A temperature of the washingwater or the finishing liquid at immersion is, for example, 10 to 70° C.The temperature should preferably be 20 to 50° C. An immersion timeshould preferably be 5 to 120 seconds, and should more preferably be 5to 15 seconds.

Meanwhile, a chemical conversion treatment liquid of the third aspect ofthe present invention contains trivalent chromium ions, cobalt ions anda sulfur compound.

In the chemical conversion treatment liquid, any chromium compoundcontaining trivalent chromium ions may be used as a source of trivalentchromium ions. For example, the source should be trivalent chromium saltsuch as chromium chloride, chromium sulfate, chromium nitrate, chromiumphosphate or chromium acetate, or, alternatively, trivalent chromiumions can be obtained by the reduction of hexavalent chromium ions ofchromic acid, a dichromate, or the like with a reducing agent. However,the source is not limited to these examples. One of the above sources oftrivalent chromium ions or any combination of at least two of them maybe used. A content of trivalent chromium ions in the chemical conversiontreatment liquid should preferably be 0.002 to 0.5 mol/l, and shouldmore preferably be 0.02 to 0.1 mol/l. Meanwhile, a concentration ofhexavalent chromium ions in the chemical conversion treatment liquidshould preferably be 1 ppm or less, and should more preferably be 0.5ppm or less.

A content of cobalt ions in the chemical conversion treatment liquid is250 ppm or less. The chemical conversion treatment liquid does notnecessarily contain cobalt ions. Since the sulfur compound is contained,the formed trivalent chromium chemical conversion coating film has asufficient corrosion resistance even when cobalt ions are not contained.A content of cobalt ions in the chemical conversion treatment liquidshould preferably be 100 to 250 ppm, and should more preferably be 150to 200 ppm, when a higher corrosion resistance is required. When thechemical conversion treatment liquid contains cobalt ions, any metalcompound containing cobalt can be used as a source of cobalt ions.Examples of such metal compounds include cobalt nitrate, cobalt sulfate,cobalt chloride, cobalt carbonate and cobalt hydroxide. However, themetal compounds are not limited to these examples. One of the abovemetal compounds or any combination of at least two of them may be used.

In the chemical conversion treatment liquid, an organic sulfur compoundis preferable as the sulfur compound. Specific examples of organicsulfur compounds include thiourea, thioglycerin, thioacetic acid,potassium thioacetate, thiodiacetic acid, 3,3-thiodipropionic acid,thiosemicarbazide, thioglycolic acid, thiodiglycolic acid, thiomaleicacid, thioacetamide, dithioglycolic acid, dithiodiglycolic acid,alkaline salts thereof, and the like. In addition, one of the abovesulfur compounds or a mixture of two or more of them can be used. Acontent of the sulfur compound in the chemical conversion treatmentliquid should preferably be 100 to 1500 ppm, should more preferably be300 to 1000 ppm, and should still more preferably be 400 to 800 ppm interms of sulfur atoms. By adding the sulfur compound, the formedtrivalent chromium chemical conversion coating film has a sufficientcorrosion resistance, even when a concentration of cobalt ions in thecoating film is 0.2 μg/cm or less, and preferably 0.17 μg/cm or less. Inaddition, in the trivalent chromium chemical conversion coating filmformed from the chemical conversion treatment liquid, hexavalentchromium which is otherwise generated in the trivalent chromium chemicalconversion coating film can be suppressed because of low concentrationof cobalt ions in the coating film.

In the chemical conversion treatment liquid, a high corrosion resistancecan be maintained even when nitrogen content in the treatment liquid isconsiderably reduced. A preferable nitrogen content is 500 ppm or less,and preferably 200 ppm or less in terms of nitrogen atoms. The contentshould more preferably be 40 to 200 ppm, and should still morepreferably be 60 to 130 ppm.

In addition, the chemical conversion treatment liquid may contain one ormore kinds of silicon compounds. Examples of silicon compounds includecolloidal silica, sodium silicate, potassium silicate, lithium silicate,and the like. However, the silicon compounds are not limited to theseexamples. When the silicon compound is contained, a concentrationthereof in the chemical conversion treatment liquid should preferably be1 to 20 g/l, and should more preferably be 2 to 10 g/l in terms of Si.Colloidal silica is particularly preferable. A concentration thereofshould preferably be 1 to 100 ml/l as a 20% SiO₂ aqueous solution. Byadding colloidal silica, a coating film with a bilayer structure of aSi—O layer and a Cr—O layer can be formed, whereby corrosion resistancecan be further improved.

In addition, the chemical conversion treatment liquid may contain one ormore kinds of agents for reducing a coating film overall frictioncoefficient. Examples of such agents for reducing a coating film overallfriction coefficient include quinoline-based compounds such as quinolinesulfonic acid, quinaldic acid, quinophthalone and derivatives thereofdescribed in Japanese Patent Application Publication No. 2005-248233.When the agent for reducing a coating film overall friction coefficientis contained, a concentration thereof in the chemical conversiontreatment liquid should preferably be 0.1 to 25 g/l, and should morepreferably be 0.2 to 15 g/l. The trivalent chromium chemical conversioncoating film according to the present invention formed by treatment withthe chemical conversion treatment liquid containing such a agent forreducing a coating film overall friction coefficient is a coating filmthat has a reduced coating film overall friction coefficient.

In addition, the chemical conversion treatment liquid may contain one ormore kinds selected from inorganic acids, alkaline salts thereof, andthe like. Examples of inorganic acids include sulfuric acid, nitricacid, hydrochloric acid, and the like. However, the inorganic acids arenot limited to these examples. When one or more kinds selected frominorganic acids, the alkaline salts thereof, and the like are contained,a concentration thereof in the chemical conversion treatment liquidshould preferably be 0.01 to 50 g/L, and should more preferably be 0.05to 20 g/L.

In addition, the chemical conversion treatment liquid may contain one ormore kinds selected from phosphorus oxoacids such as hypophosphorousacid, phosphoric acid, alkaline salts thereof, and the like. When one ormore kinds selected from phosphorus oxoacids such as hypophosphorousacid, phosphoric acid, alkaline salts thereof, and the like arecontained, the concentration thereof in the chemical conversiontreatment liquid should preferably be 0.1 to 50 g/L, and should morepreferably be 4 to 25 g/L.

Furthermore, the chemical conversion treatment liquid may contain one ormore kinds of hydroxycarboxylic acids, monocarboxylic acids, polyvalentcarboxylic acids, aminocarboxylic acids, alkaline salts thereof, and thelike as chelating agents for trivalent chromium ions. Examples ofhydroxycarboxylic acids include malic acid, and the like. However, thehydroxycarboxylic acids are not limited to these examples. Examples ofmonocarboxylic acids include formic acid, acetic acid, and the like.However, the monocarboxylic acids are not limited to these examples.Examples of polyvalent carboxylic acids include: dicarboxylic acids suchas oxalic acid, malonic acid, succinic acid, adipic acid and diglycolicacid; tricarboxylic acids such as propanetricarboxylic acid, and thelike. However, the polyvalent carboxylic acids are not limited to theseexamples. Examples of aminocarboxylic acids include glycine, asparticacid, and the like. However, the aminocarboxylic acids are not limitedto these examples. Among these, polyvalent carboxylic acids arepreferable, and oxalic acid, malonic acid and succinic acid are morepreferable. When the above carboxylic acids and alkaline salts thereofare contained, a concentration thereof in the chemical conversiontreatment liquid should preferably be in the range of 0.2 to 2 mole,should more preferably be in the range of 0.3 to 2 mole, should stillmore preferably be in the range of 0.5 to 2 mole, and should stillfurther more preferably be in the range of 0.7 to 1.8 mole per mole oftrivalent chromium ions.

In addition, the chemical conversion treatment liquid may contain one ormore kinds of ions of metal selected from Mg, Al, Mn, Ti, W, V, Mo, Ni,Fe, Zn, Zr, Ca, Nb, Ta, Sn and Ce. When the metal ions are contained, aconcentration thereof in the chemical conversion treatment liquid shouldpreferably be 1 to 10 g/l, and should more preferably be 2 to 8 g/l.

A pH of the chemical conversion treatment liquid should preferably be0.5 to 5, and should more preferably be 2 to 3. The pH may be adjustedto this range by using the inorganic acid ions as described below, or byusing an alkaline agent such as an alkaline hydroxide, ammonia water, orthe like.

The rest of the chemical conversion treatment liquid other than theabove essential components is water.

A method for forming a trivalent chromium chemical conversion coatingfilm on zinc or zinc alloy plating by using the chemical conversiontreatment liquid is commonly to immerse a zinc or zinc alloy platedsubstrate into the chemical conversion treatment liquid. A temperatureof the chemical conversion treatment liquid at immersion is, forexample, 10 to 70° C. The temperature should preferably be 25 to 35° C.An immersion time should preferably be 5 to 600 seconds, and should morepreferably be 15 to 120 seconds. Meanwhile, immersion into a dilutednitric acid solution, a diluted sulfuric acid solution, a dilutedhydrochloric acid solution, a diluted hydrofluoric acid solution, or thelike may be performed before trivalent chromium chemical conversiontreatment, for activating the surface of the zinc or zinc alloy plating.The conditions and treatment operations other than those described abovemay follow the conventional hexavalent chromate treatment method.

Overcoating the trivalent chromium chemical conversion coating filmformed by using the chemical conversion treatment liquid according tothe present invention can improve the corrosion resistance thereof, andthus is a highly effective means for achieving longer-lasting corrosionresistance. For example, the trivalent chromium chemical conversioncoating film is firstly formed on the zinc or zinc alloy plating usingthe chemical conversion treatment liquid according to the presentinvention, then washed with water, then immersed into an overcoatingsolution or subjected to an electrolytic treatment therein, andthereafter dried. Alternatively, the trivalent chromium chemicalconversion coating film may be dried after formation thereof, andthereafter further immersed into an overcoating solution or subjected toan electrolytic treatment therein, and then dried. Here, as theovercoating, as well as an inorganic coating film made of silicates,phosphates, or the like, an organic coating film made of polyethylene,polyvinyl chloride, polystyrene, polypropylene, methacrylate resin,polycarbonate, polyamide, polyacetal, fluorine resin, urea resin,phenolic resin, unsaturated polyester resin, polyurethane, alkyd resin,epoxy resin, melamine resin, or the like may be effectively used.

As the overcoating solution for overcoating such a film, DIPCOAT W orCC445 available from Dipsol Chemicals Co., Ltd. or the like may be used.The thickness of the overcoating may be any value, but should preferablybe 0.1 to 30 μm.

Next, the present invention is described by referring to Examples andComparative Examples.

EXAMPLES Examples 1 to 8

An M6 bolt (material: iron), which had been plated with zinc using azincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in Examples 1to 2 and 6 to 8 or acidic zinc (EZ-960 available from Dipsol ChemicalsCo., Ltd.) in Examples 3 to 5 in a thickness of 8 μm, was immersed in achemical conversion treatment liquid shown in Table 1 under conditionsshown in Table 1. In addition, in Examples 6 to 8, the immersed bolt wasimmersed in a finishing liquid shown in Table 1 under conditions shownin Table 1. After immersion, the coating film was dried under conditionsat 80° C. for 10 minutes.

TABLE 1 Example 1 2 3 4 5 6 7 8 Composition of trivalent chromiumchemical conversion treatment liquid Cr³⁻ (mol/l) 0.077 0.077 0.0770.077 0.077 0.077 0.077 0.077 Cr⁶⁺ 0 0 0 0 0 0 0 0 Oxalic acid (g/l) 1212 12 12 12 12 12 12 (mol/mol of Cr³⁺) (1.7) (1.7) (1.7) (1.7) (1.7)(1.7) (1.7) (1.7) Co²⁺ (mol/l) 0.034 0.034 0.034 0.034 0.034 0.034 0.0340.034 hexavalent chromium generation Tannic Gallic Thiourea Vanadium ionphosphate ion — — — suppressing agent (0.5 g/l) acid acid (vanadium(sodium chloride) dihydrogen phosphate) Nitrogen content in terms of 4.24.2 4.2 4.2 4.2 4.2 4.2 4.2 nitrogen atoms (g/l) pH 2.3 2.3 2.3 2.3 2.32.3 2.3 2.3 Treatment temperature (° C.) 30 30 30 30 30 30 30 30Treatment time (seconds) 40 40 40 40 40 40 40 40 Composition offinishing liquid Ascorbic acid (g/l) 2 Tannic acid (g/l) 1 Chromiumphosphate (g/l) + 2 + 3 citric acid (g/l) pH of finishing liquid 4 4 6Finishing treatment 25 40 25 temperature (° C.) Finishing treatment 15 515 time (seconds)

A 40% chromium nitrate aqueous solution was employed as a source ofCr³⁺, and cobalt nitrate was employed as a source of Co²⁺. The rest ofthe solution was water.

Comparative Example 1

An M6 bolt (material: iron), which had been plated with zinc using azincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in athickness of 8 μm, was subjected to a hexavalent chromate treatment. Asthe hexavalent chromate treatment liquid, Z-493 (10 ml/l) available fromDipsol Chemicals Co., Ltd. was used, and the bolt was immersed at 25° C.for 20 seconds. After immersion, the coating film was dried underconditions at 60° C. for 10 minutes.

Comparative Example 2

A trivalent chromium chemical conversion coating film was formed on anM6 bolt (material: iron), which had been plated with zinc using azincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in athickness of 8 um. As the chemical conversion treatment liquid, achemical conversion treatment liquid having the following compositionwas used, and the bolt was immersed at 30° C. for 40 seconds. Afterimmersion, the coating film was dried under conditions at 80° C. for 10minutes.

Cr³⁺ 4 g/l (40% chromium nitrate was used. 0.077 mol/l in terms of Cr)Oxalic acid 12 g/l Co(NO₃)₂ 10 g/l (0.034 mol/l in terms of Co) pH 2.3

Table 2 shows concentrations of Cr³⁺, concentrations of Cr⁶⁺ andconcentrations of Co²⁺ in the chemical conversion coating films obtainedin Examples 1 to 8 and Comparative Examples 1 and 2, appearances,results of salt spray test (JIS Z-2371) and amounts of hexavalentchromium eluted after the storage test. As shown in Table 3, the coatingfilms of Examples 1 to 8 exhibited corrosion resistance equal to orbetter than that of the conventional hexavalent chromate chemicalconversion coating film of Comparative Example 1. In addition, theamounts of hexavalent chromium eluted after storage test were less thanthe measurement limit value.

TABLE 2 Corrosion resistance Amount of Concentration in Time requiredfor Cr⁶⁺ eluted coating film (μg/cm²) Appearance of the formation ofafter shelf Cr³⁺ Cr⁶⁺ Co²⁺ coating film white rust (Hrs) test (μg/cm²)Example 1 8.5 0 0.7 Pale reddish green 300 <0.05 Example 2 10.5 0 2.5Pale reddish green 300 <0.05 Example 3 6.7 0 1.6 Pale reddish green 300<0.05 Example 4 7.0 0 0.6 Pale reddish green 300 <0.05 Example 5 5.4 00.4 Pale reddish green 300 <0.05 Example 6 10.0 0 2.3 Pale reddish green300 <0.05 Example 7 10.8 0 1.0 Pale reddish green 300 <0.05 Example 810.7 0 2.9 Pale reddish green 300 <0.05 Comparative 20 6.8 0 Reddishgreen 240 6.8 Example 1 Comparative 11 0.01 2.8 Pale reddish green 2400.12 Example 2

Examples 9 to 11

An M6 Bolt (material: iron), which had been plated with zinc using azincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in athickness of 8 μm, were immersed into a chemical conversion treatmentliquid shown in Table 3 under conditions shown in Table 3. Afterimmersion, the coating film was dried under conditions at 80° C. for 10minutes.

TABLE 3 Example 9 10 11 Cr³⁺ (mol/l) 0.038 0.038 0.038 Cr⁶⁺ (ppm) 0 0 0Nitrogen content in terms of 90 90 90 nitrogen atoms (ppm) SO₄ ²⁻ (g/L)0 6.0 6.0 Cl⁻ (g/L) 4.4 0 0 Thiodiglycolic acid (g/L) 2 (430) 2 (430) 0(in terms of sulfur content (ppm)) Thiourea (g/L) 0 0 2 (840) (in termsof sulfur content (ppm)) Co²⁺ (ppm) 200 200 200 Si (g/L) 2 2 2 ph oftreatment liquid 2.4 2.4 2.4 Treatment temperature (° C.) 30 30 30Treatment time (seconds) 60 40 40

A 35% chromium chloride aqueous solution (Example 9) or a 35% chromiumsulfate aqueous solution (Examples 10 and 11) was employed as a sourceof Cr³⁺. Cobalt chloride (Example 9) or cobalt sulfate (Examples 10 and11) was employed as a source of Co²⁺. Si was an acidic colloidal silica(SNOWTEX-O available from Nissan Chemical Industries, Ltd.). The rest ofthe solution was water.

Examples 12 to 14

Overcoating was performed on the trivalent chromium chemical conversioncoating film of Example 9. Table 4 shows the overcoating conditions.

TABLE 4 Example 12 13 14 Type of Chromium Inorganic Methacrylateovercoating phosphate-based silicate-based resin-based inorganicinorganic Si-dispersed-type coating film coating film organic coatingfilm Treatment 150 ml/l 200 ml/l Undiluted liquid concentration was usedTreatment 45° C., 25° C., 25° C., conditions 10 seconds 30 seconds 30seconds Name of agent ZTB-118 CC-445Y DIPCOAT W available from availablefrom available from Dipsol Chemicals Dipsol Chemicals Dipsol ChemicalsCo., Ltd. Co., Ltd. Co., Ltd.

Table 5 shows concentrations of Cr³⁺, concentrations of Cr⁶⁺ andconcentrations of Co²⁺ in the chemical conversion coating films obtainedin Examples 9 to 14 and Comparative Examples 1 and 2, appearances,results of salt spray test (JIS Z-2371) and amounts of hexavalentchromium eluted after storage test. As shown in Table 5, the coatingfilms of Examples 9 to 14 exhibited corrosion resistance equal to orbetter than that of the conventional hexavalent chromate chemicalconversion coating film of Comparative Example 1. In addition, theovercoated coating films (Examples 12 to 14) exhibited better corrosionresistance than the conventional hexavalent chromate chemical conversioncoating film. In addition, the amounts of hexavalent chromium elutedafter storage test were less than the measurement limit value.

TABLE 5 Corrosion resistance Amount of Concentration in Time requiredfor Cr⁶ ⁺ eluted coating film (μg/cm²) Appearance of the formation ofafter shelf Cr³⁺ Cr⁶⁺ Co²⁺ coating film white rust (Hrs) test (μg/cm²)Example 9 7.0 0 0.1 Light blue 240 <0.05 Example 10 5.8 0 0.1 Light blue240 <0.05 Example 11 6.0 0 0.1 Light blue 240 <0.05 Example 12 7.0 0 0.1light interference 1000 or more <0.05 color Example 13 7.0 0 0.1colorless 1000 or more <0.05 Example 14 7.0 0 0.1 colorless 1000 or more<0.05 Comparative 20 6.8 0 Reddish green 240 6.8 Example 1 Comparative11 0.11 2.8 Pale reddish green 240 0.12 Example 2

(Measurement of Cr⁶⁺ Concentration in Coating Film)

A coating film sample (50 cm²) was immersed into approximately 50 ml ofhot water at a temperature of 100° C. for 10 minutes. An amount ofhexavalent chromium eluted from the coating film sample was determinedby the absorption spectroscopy using diphenylcarbazide (in accordancewith EN-15205).

(Measurement of Cr³⁺ Concentration and Co²⁺ Concentration in CoatingFilm)

After measuring the Cr⁶⁺ concentration, the same sample was dissolvedinto hydrochloric acid, and Cr³⁺ and Co²⁺ concentrations in the solutionwas measured by ICP optical emission spectrometry.

(Salt Spray Test)

In a salt spray test, evaluation was made in accordance with JIS-Z-2371.

(Procedure of Storage Test and Measurement of Amount of HexavalentChromium Eluted after the Storage Test)

A storage test was performed as an acceleration test by employing amethod in which a sample for the elution test was left for 30 days in aconstant temperature and humidity chamber maintained at a temperature of80° C. and a humidity of 95%. Then, the sample after the storage testwas immersed into hot water at a temperature of 100° C. for 10 minutesby a method similar to the above-described measuring method of Cr⁶⁺concentration in a coating film. An amount of hexavalent chromium elutedfrom the coating film sample was determined by absorption spectroscopyusing diphenylcarbazide (in accordance with EN-15205).

Examples 15 to 20

An M6 Bolt (material: iron), which had been plated with zinc using azincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in athickness of 8 μm, was immersed into a chemical conversion treatmentliquid shown in Table 6 under conditions shown in Table 6. Afterimmersion, the coating film was dried under conditions at 80° C. for 10minutes.

TABLE 6 Example 15 16 17 18 19 20 Cr³⁺ (mol/l) 0.038 0.038 0.038 0.0380.038 0.038 Cr⁶⁺ (ppm) 0 0 0 0 0 0 Nitrogen content in 135 270 270 90 9090 terms of nitrogen atoms (ppm) Tartaric acid (g/l) 0 0 0 2 (0.35) 2(0.35) 0 (mol/mol of Cr³⁺) Malic acid(g/l) 5 (0.97) 5 (0.97) 0 0 0 5(0.97) (mol/mol of Cr³⁺) SO₄ ²⁻ (g/l) 0 0 2 2 0 0 Cl⁻ (g/l) 4 4 4 4 4 4Co²⁺ (mol/l) 0.015 0.008 0 0.008 0.008 0.008 VOSO₄ (g/l) 1 1 0 0 1 0Ti(SO₄)₂ 0 0 1 0 0 1 MgSO₄ (g/l) 0 0 0 2 0 0 Si (g/l) 5 2 5 10 5 10 phof treatment 2.0 2.1 2.0 2.3 2.4 2.5 liquid Treatment temperature 40 4030 30 30 30 (° C.) Treatment time 20 30 60 40 40 60 (seconds)

A 35% chromium chloride aqueous solution was employed as a source ofCr³⁺. Cobalt chloride was employed as a source of Co²⁺. Si was an acidiccolloidal silica (SNOWTEX-O available from Nissan Chemical Industries,Ltd.). The rest of the solution was water. Note that, the nitrogencontent was derived from NO₃ ⁻.

TABLE 7 Corrosion resistance Amount of Concentration in Time requiredfor Cr⁶⁺ eluted coating film (μg/cm²) Appearance of the formation ofafter shelf Cr³⁺ Cr⁶⁺ Co²⁺ coating film white rust (Hrs) test (μg/cm²)Example 15 7.2 0 0.15 Light blue 240 <0.05 Example 16 7.0 0 0.1 Lightblue 240 <0.05 Example 17 6.2 0 0 Light blue 168 <0.05 Example 18 5.0 00.15 Light blue 144 <0.05 Example 19 4.3 0 0.1 Light blue 144 <0.05Example 20 4.0 0 0.1 Light blue 144 <0.05

1-19. (canceled) 20-23. (canceled)
 24. A chemical conversion treatmentliquid for forming a trivalent chromium chemical conversion coating filmon zinc or zinc alloy plating, characterized in that a content oftrivalent chromium ions in the treatment liquid is 0.002 to 0.5 mol/l, aconcentration of hexavalent chromium ions is 1 ppm or less, a content ofcobalt ions is 0.1 mol/1 or less, a hexavalent chromium generationsuppressing agent that can suppress generation of hexavalent chromiumwhich is generated in the trivalent chromium chemical conversion coatingfilm is contained in said liquid, and a pH of the chemical conversiontreatment liquid is 0.5 to
 5. 25. The chemical conversion treatmentliquid according to claim 24, wherein the hexavalent chromium generationsuppressing agent is within the range of 0.1 to 5 g/l.
 26. The chemicalconversion treatment liquid according to claim 24, wherein thehexavalent chromium generation suppressing agent is a reducing compound.27. The chemical conversion treatment liquid according to claim 24,wherein the hexavalent chromium generation suppressing agent is selectedfrom the group consisting of a tannic acid and a salt thereof, gallicacid and a salt thereof, tartaric acid and a salt thereof, citric acidand a salt thereof, ascorbic acid and a salt thereof, a vanadiumcompound, a titanium compound, phosphoric acid and a salt thereof, andchromium phosphate.
 28. The chemical conversion treatment liquidaccording to claim 24, further comprising a chelating agent in the rangeof 0.2 to 2 mole per mole of Cr3+.
 29. The chemical conversion treatmentliquid according to claim 24, wherein a content of nitrogen is 500 ppmor less in terms of nitrogen atoms.
 30. The chemical conversiontreatment liquid according to claim 24, further comprising aquinoline-based compound or a derivative thereof.
 31. Washing water or afinishing liquid for a trivalent chromium chemical conversion coatingfilm formed on zinc plating or zinc alloy plating, characterized bycomprising a hexavalent chromium generation suppressing agent that cansuppress generation of hexavalent chromium which is generated in thetrivalent chromium chemical conversion coating film at 0.1 to 10 g/l,wherein a pH of the washing water or the finishing liquid is 2 to 10.32. A chemical conversion treatment liquid for forming a trivalentchromium chemical conversion coating film on zinc or zinc alloy plating,characterized in that a content of trivalent chromium ions in thetreatment liquid is 0.002 to 0.5 mol/l, a concentration of hexavalentchromium ions is 1 ppm or less, a content of cobalt ions is 250 ppm orless, and a sulfur compound is contained in the range of 100 to 1500 ppmin terms of sulfur atoms.
 33. The chemical conversion treatment liquidaccording to claim 32, wherein a content of nitrogen is 500 ppm or lessin terms of nitrogen atoms.
 34. The chemical conversion treatment liquidaccording to claim 32, further comprising a silicon compound.
 35. Thechemical conversion treatment liquid according to claim 32, furthercomprising a quinoline-based compound or a derivative thereof. 36-37.(canceled)
 38. The trivalent chromium chemical conversion coating film,wherein a corrosion resistance (a time required for the formation ofwhite rust) in a salt spray test is 96 hours or more, a concentration ofhexavalent chromium in terms of metal atoms in the chemical conversioncoating film is less than 0.01 μg/cm2, and an amount of hexavalentchromium eluted from the coating film left for 30 days in a constanttemperature and humidity chamber at a temperature of 80° C. and at ahumidity of 95% (an amount eluted by immersion of the coating film intohot water at a temperature of 100° C. for 10 minutes) is less than 0.05μg/cm2, wherein the trivalent chromium chemical conversion coating filmis formed on zinc or zinc alloy plating, and after the formationthereof, the coating film is treated with the washing water or thefinishing liquid; comprising: a hexavalent chromium generationsuppressing agent that can suppress generation of hexavalent chromiumwhich is generated in the trivalent chromium chemical conversion coatingfilm at 0.1 to 10 g/l, wherein a pH of the washing water or thefinishing liquid is 2 to 10.